Troubleshooting an Uneven Spoilboard

A CNC owner has trouble getting a uniform flat surface on his spoilboard. December 11, 2012

QuestionI've been trying to figure this out for a while, but can't. Every time we resurface the spoil board, it is uneven in the X direction.

Forum Responses(CNC Forum)
From contributor P:What do you mean by uneven? Is your spindle perpendicular to the table? Is the spoil board changing with humidity change? Are both sides of the spoil board surfaced?

From contributor B:It depends what you mean by "uneven." If the surface is uneven in a regular pattern, check to see that the spindle is square to the table. That is to say the fly cutter may be tilted at a slight angle.
From contributor M:Contributor B is correct. You will need to check which direction the spindle is out of tram, and by how much to correct the problem. It is not hard to do yourself, but you will need someone who has done it before to walk you through it. You will also need a dial indicator and a solid method of attaching it to the spindle. Hopefully your machine allows easy access to the spindle mounting bolts.
From the original questioner:After we surface the spoil board, the table is off by about .007 from one side to the other in the X direction. I'm not sure but I think it might have something to do with the surfacing tool. We run a 2 1/2 southeast tool surfacing bit at 350' ppm and. It runs up and down the Y stepping off to the X at the end of each pass. The side of the table that the cutter starts on is the deeper side. As it works its way over I think the cutters start to dull out and it is forced up.
From contributor K:Been there, seen that, had to adjust my (ex) Multicam for that. Here's a very easy way of checking squareness of spindle. Take a piece of steel wire of about 4mm, and some 25 cm long. Put in place of bit, bend it sideways and the end down again. Position the end just above the table surface. Rotate the spindle by hand and look at the height of the wire end.
From contributor B:With the thickness of the spoil board varying across the overall x dimension width, I would do two things. First I'd rewrite the fly cut file to start at the other end of the x-axis. This will test your theory. If it is the bit getting dull and rising, then the opposite side of the spoil board will be thicker.

If that test reverses the thickness, you've solved your problem. If not, I think you need to look at the alignment between your machine table (not the spoil board) and the bridge. You can do that by removing the spoil board and touching down to the table frame at x-low and then at x-high to see if there is a difference.

From contributor A:Keep this thing in perspective. Your cuts are off.3 degrees? That means your deviation in a 3/4" piece is .004". If you have to, get some shim stock and use contributor K's method.

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